An organic light-emitting diode device, also called an OLED or an organic electroluminescent (EL) device, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), an organic light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing angle and capability for full color displays and for other applications. Tang, et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211. Since then, numerous OLEDs with alternative layer structures and using many different types of organic EL materials have been disclosed.
An OLED is actually a current driven device. Its luminance is proportional to current density, but its lifetime is inversely proportional to current density. In order to achieve high brightness, an OLED has to be operated at a relatively high current density, but this will result in a short lifetime. Thus, it is critical to improve the luminous efficiency of an OLED while operating at the lowest possible current density.
In order to dramatically improve luminous efficiency and to increase the lifetime for OLEDs, a tandem OLED (or stacked OLED, or cascaded OLED) structure, which is fabricated by stacking several individual OLEDs vertically and driven by only a single power source, has been proposed or fabricated. See U.S. Pat. Nos. 6,337,492, 6,107,734, 6,717,358, JP Patent Publication 2003-045676, U.S. Patent Publications 2003/0189401 A1, and 2003/0170491 A1. In a tandem OLED having a number of N EL units, the luminous efficiency can be N times as high as that of a conventional OLED containing only one EL unit. Therefore, the tandem OLED needs only about 1/N of the current density used in a conventional OLED to achieve the same luminance. As a result, the lifetime of the tandem OLED will be about N times that of a conventional OLED. However, the drive voltage will be about N times as high as that of a conventional OLED at the same current density.
In display applications, there is a voltage limit for the circuitry. For example, in an active matrix OLED display based on a thin film transistor (TFT) backplane or drive circuitry, the maximum permitted voltage drop across the OLED component can be less than 12 volts (V). A tandem OLED structure may not be suitable for use in display applications if its drive voltage is higher than 12 V to produce an intended luminance. Therefore, it is important to make a tandem OLED with a drive voltage suitable for display applications. However, the aforementioned prior art does not disclose a tandem OLED containing more than two EL units and with a drive voltage suitable for the common TFT drive circuitry.